1. Field of the Invention
The present invention relates to a transcoder for converting the coding format of an image signal and to an imaging apparatus and signal processor using this transcoder.
2. Description of the Related Art
Prior Art
To record moving image signals on recording media or transmitting them over the Internet, radio and other communication media, techniques for compressing and encoding moving image signals by MPEG and other compressing algorithms have recently been used in various fields. For example, MPEG2-Video (ISO/IEC13818-2) is used for recording to and playing back from DVDs. In addition, MPEG4-Video (ISO/IEC14496-2) is used in radio communication and Internet applications. Including them, a variety of encoding methods have been proposed. With the diversification of encoding methods, however, it has become necessary to transcode data among these formats and improve the transcoding capability. In particular, if the time required for the transcoding process is reduced, the user side is given raised convenience:
A technique disclosed in Japanese Patent Laid-open No. 2002-152755 concerns improvement in the transcoding speed. This technique “reduces the temporal resolution of an input video stream by thinning out bidirectional predictive frames and reduces the spatial resolution by reducing the picture size of the temporal resolution-lowered video stream without inversely transforming transform coefficients” aimed at “shortening the transcoding time required to convert the video data to a different kind of video data by entirely eliminating the necessity of inverse transformation of coded transform coefficients in the frequency domain”.